1. Field of the Invention
This invention relates in general to satellite communications systems. More particularly, this invention relates to the generation of basic spot beams for multiple-beam satellite systems, and a method for dividing the basic spot beam that has high e.g. 3–4 dB gain drop into a number of smaller sub-beams with a low e.g. 1 dB gain drop, using the existing frequencies of the basic spot beams.
2. Background of the Related Art
A phased array antenna generally includes a collection of radiating elements closely arranged in a predetermined pattern and energized to produce beams in specific directions. When elements are combined in an array, constructive radiation interference results in a main beam of concentrated radiation, while destructive radiation interference outside the main beam reduces stray radiation.
In satellite communications systems as illustrated in FIG. 5, signals are typically beamed between satellites 100 and fixed coverage region(s) on the Earth 102 using these beams of concentrated radiation. Bandwidth is often a limited resource, and has to be used efficiently. In order to cover such large regions and reuse the same frequency, the use of a multiple beam phased array antenna 101 has been discovered to be an effective solution. Beam coverage from the phased array antenna 101 is accomplished by producing a number of spot beams 104 directed towards specific areas 103 of the coverage region. These spot beams 104 are generated by energizing radiating elements of the phased array antenna with selected amplitudes and phases, and can be realized by the use of a Digital Beam Former (DBF).
The DBF operates in conjunction with the phased array antenna to realize the multibeam coverage by applying appropriate phase shifts to each of the radiating elements in the array.
To completely cover a satellite service area with spot beams, it is customary to use a network of contiguous beams, where each beam is defined at a contour level of 3- to 4-dB from the beam peaks. The phased array antenna is therefore sized to accommodate the peak gains required in order to satisfy these edge gain requirements, which results in a large aperture for the phased array antenna due to the fact that aperture size is directly related to the peak gain requirements.
This large aperture that is needed to achieve the high peak gain at the scanned position will have to be populated with a large number of radiating elements. The excessive number of antenna elements, combined with the large number of beams, make the beam former very complex, with high power and mass requirements. This is due to the fact that the complexity, mass, and power requirements of the digital beam former are increased depending principally upon the number of elements, and are much less dependent on the number of beams.